How a Pedigree Chart Works
A pedigree, also known as a family tree, keeps track of prodigies of a family from two or more generations. This type of diagram is very useful for geneticists to examine families that have prodigies that are disease carriers or affected by the disease. Using the pedigree chart, geneticists can determine if the disease is dominant or recessive depending on the number of prodigies or parents that are carriers or affected. Researchers would then attempt to determine the genotypes of the parents and prodigies, which takes time because they would have to determine if the prodigies were heterozygous recessive, autosomal dominant/recessive, sex-linked recessive, or homozygous dominant/recessive. In addition, pedigrees can reveal how rare a disorder is. In a pedigree chart, geneticists use symbols to represent prodigies.
In pedigrees, a square represents a male and a circle represents a female. A line that connects a square and a circle represents mating, in other words, married couples. A line that branches away from the line connecting the parents represents parents with children in which the prodigies are drawn by order of birth. If two lines are drawn to two prodigies and forms a triangle in which a line connects both prodigies, then the children are identical twins, or monozygotic. If two lines are drawn to two prodigies but does not have a line connecting both prodigies, then the children are fraternal twins, or dizygotic. A circle or square that is completely darkened represents an affected prodigy. A circle or square that is half-black is a heterozygote for an autosomal recessive disorder. A circle with a dot in the middle represents a sex-linked recessive carrier. A slash on a circle or square represents a deceased prodigy. An arrow pointing to a circle or square signifies a propositus, or a prodigy of interest to the geneticist. If two lines connect to a circle and a square, then the marriage is consanguineous, meaning same blood.
Geneticists would take consideration of a family’s generations when they compose a pedigree chart. From there, they can examine the disease for hybrid crosses by previous generations based on the pedigree. For example, geneticists can use Punnett squares to determine a cross, such as a monohybrid cross. Diseases can be sex-linked or autosomal, which researchers have to interpret by looking at the pedigree. With these pedigrees, geneticists hope that they can provide help to future families that are carriers or affected by a disorder.